A radiant heater is a heater that is designed to transmit much of its heat energy by line of sight radiation rather than by convection. A typical radiant heater may use an open-wire, quartz jacket or flat panel heating element that is much hotter than the ambient temperature of the room. Recently, quartz jacketed heater elements containing a halogen gas and which emit more of their energy in the visible wavelengths have begun to replace conventional open-wire heating elements which emit more of their energy as infrared. The use of heater elements radiating their energy from a compact volume makes possible a more effective focusing of the radiant energy and, accordingly, radiant heaters using halogen heater elements tend to employ parabolic reflectors to focus their heat energy into a beam. The use of focused heating has allowed radiant heaters to operate at lower power levels while maintaining a comfortable level of heating.
While the use of a reflector materially improves the efficiency of heating the objects or persons in the beam path directly in front of the heater, it is necessary to reduce the likelihood that the intense heat radiated could cause a flammable object positioned too close to the front of the heater to overheat or burst into flame. Among the well-known overheat protection tests are the Canadian Standards Association (CSA) test in which a cheese cloth is used as the flammable object and the Underwriter's Laboratories standard UL 1278 which uses terry cloth towel material draped over different portions of the front of the heater. The use of terry cloth is a more severe overheat protection test than the use of cheese cloth because terry cloth offers more exposed fiber ends, is less permeable and subjects a greater effective area of flammable material to the heat source than does open-weave cheese cloth. Since terry cloth chars at about 200.degree. C., a safety device must cut off electrical power before any local area of the terry cloth attains this temperature.
Since it is obviously impractical for a thermostat to directly sense the surface temperature of a flammable object that might be placed in front of the heater, indirect sensing is required. Portable radiant heaters employing halogen elements pose a particular difficulty in passing the draped terry cloth overheat protection test because they emit so concentrated a beam of light that a small spot on the terry cloth may receive sufficient radiant energy to catch fire. Conventional thermostats have not been effective in detecting the type of local hot spot produced during the drape test because, among other reasons, they are incapable of directly sensing temperature over the entire frontal area of the heater.
Heretofore, the only thermostatic technique which has allowed a high-intensity radiant heater to pass the terry cloth overheat protection test has required the use of a capillary tube thermostat as shown in U.S. Pat. No. 5,381,509 issued Jan. 10, 1995 and assigned to the W.B. Marvin Mfg. Co. The aforementioned patent teaches that a capillary thermostat should have its sensing tube positioned to extend across the front grille of the heater so that it will sense the temperature of the air heated by the surface of the object upon which the radiation from the heater is directed. While the capillary tube thermostat is effective as a overheat protection device, it is an expensive component compared to the cost of the other heater components.